Potato-shaped graphite particles with low impurity rate at the surface, method for preparing the same

ABSTRACT

Modified graphite particles obtained from graphite or based on graphite, the said particles having impurities in their internal structure and having on the surface a low, even nil, rate of an impurity or several impurities. In addition, these particles have at least one of the following characteristics:
         a tab density between 0.3 and 1.5 g/cc;   a potatolike shape; and   a granulometric dispersion such that the D90/D10 ratio varies between 2 and 5 and the particles have a size between 1 and 50 μm.       

     These particles can be used for fuel cells, electrochemical generators, or as moisture absorbers and/or oxygen absorbers and they have important electrochemical properties. The electrochemical cells and batteries thus obtained are stable and safe.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/857,861, filed on Apr. 5, 2013, which is a continuation of U.S.application Ser. No. 12/572,510, filed on Oct. 2, 2009, which is acontinuation of U.S. application Ser. No. 11/606,231, filed Nov. 30,2006, which is a continuation of U.S. application Ser. No. 10/381,843,filed Sep. 26, 2003, which was the National Stage of InternationalApplication No. PCT/CA01/01511, filed Oct. 24, 2001. This applicationalso claims foreign priority to Canada 2,324,431, filed on Oct. 25,2000. The entire contents of each U.S. application Ser. No. 13/857,861,U.S. application Ser. No. 12/572,510, U.S. application Ser. No.11/606,231, U.S. application Ser. No. 10/381,843, InternationalApplication No. PCT/CA01/01511, and Canada 2,324,431 of are herebyincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to modified graphite particles and toparticles based on graphite that are also characterized in that theyhave a potatolike shape. The present invention also concerns processesthat make it possible to prepare these new particles, as well as the useof the particles thus obtained in particular as moisture absorbersand/or oxygen absorbers. These processes can be monitored by controllingthe values obtained from the mathematical functions that arecharacteristic of the shape of the crystalline (edge, basal, Lc and La)and geometric structures of the graphite. These new particles alsoexhibit, within the scope of an electrochemical application, improvedstability to cycling by increasing, on one hand, the density of theelectrode and on the other, the diffusion kinetics of the intercalingmaterial (Li, Na or other).

BACKGROUND ART

Graphite has important electrochemical properties. Thus for naturalgraphites, which are available abundantly in nature, a reversiblecapacity of 372 mAh/g and a voltage plateau close to that of lithiumhave been established.

Graphite has been introduced into commercial lithium-ion batteries, asmentioned in the patent granted to Sanyo in the United States, undernumber U.S. Pat. No. 5,882,818. Its important characteristics, inaddition to its low cost price, have made natural graphite a goodcandidate as anode in lithium-ion batteries. However, coating of uniformelectrodes remains problematic due to the physical shape of theseparticles, which have the shape of flakes. For this reason, carrying outthe coating effectively requires an additional calendering step when theelectrodes for Li-ion batteries are manufactured (Energy Storage Systemsfor Electronics, by Testuya Osaka and Madhav Datta, (2000), page 125).The compactness density is low, which results in electrodes havinggreater thicknesses. The performances of the anode depend on the type ofgraphite and the physical shape of these particles. The efficiency ofthe first intercalation of the ion in graphite is dependent on thespecific surface area and the edge surface fraction (K. Zaghib et al, J.Electrochemical Soc. 147 (6) 2110 to 2115, 2000). A low specific surfacearea is associated with a lower contribution of passivation film.

Natural graphite is found exclusively in the form of flakes, whileartificial graphite can be found in the form of flakes, fibers orspheres. The flake shape has an elevated degree of preferentialorientation which will induce anisotropy in the electrode. Anorientation such as this reduces the intercalation kinetics of lithiumacross the edges. However, the only spherical carbon available on themarket is Mesocarbon Microbeads MCMB processed at 2,800° C. by Osaka Gas(T. Kasuh et al., J. Power Source 68 (1997), 99). This carbon is anartificial graphite that requires costly processing at high temperatureto be ordered, as well as complex synthesis that can increase itsproduction cost. The maximum reversible capacity obtained with thisartificial graphite is of the order of 280 mAh/g, which is low incomparison to the corresponding capacity of natural graphite, which is372 mAh/g.

U.S. Pat. No. 6,139,990 of Kansai Netsukkagaku Kabushiki Kaisha grantedon Oct. 31, 2000, describes graphite particles that are modified androunded, having an almost spherical form, characterized in that theirdegree of circularity is greater than or equal to 0.86 and in that,using X-ray diffraction measurement, the peak of the intensityrelationship between one face 002 (parallel to the graphite layers) andface 110 (perpendicular to the graphite layers), that serve as therandom orientation index, must not be less than 0.0050. These particleshave poor homogeneity as regards their granulometric distribution, whichlimits their use in electrochemical cells, especially with propylenecarbonate as electrolyte. This represents a major disadvantage forlow-temperature applications. A lack of safety of electrochemical cellsincorporating such graphite particles is also noted.

Patent application EP-A-0,916,618 filed in the name of the OSAKA GAS Co.Ltd., on the other hand, describes a graphite material in which theformation of cavities has been optimized in order to increase theelectrochemical capacity of the electrodes containing it. While thesematerials have become interesting with regard to their use inprimary-type batteries, they are of little interest for otherelectrochemical applications, in particular because of the fragility oftheir structure and the resulting lack of stability for capacitiesgreater than 400 mA/g.

The handling required for converting natural graphite into sphericalgraphite presents net advantages in comparison to the standard naturalgraphite present in the form of flakes, as well as in comparison tospherical artificial graphite (MCMB).

Thus a need existed for graphite-based particles in a stable form thatcan be compressed easily to the point of obtaining an elevated density,these particles presenting electrochemical capacities and anisotropiesthat are greater than or equal to those of the known forms of graphiteparticles. In particular, these particles make it possible to producehomogeneous and compact electrodes and also promote the use of PC.

SUMMARY OF THE INVENTION

The present invention especially concerns potatolike shaped modifiedgraphite particles having impurities in their internal structure andhaving on the surface a low, even nil, rate of an impurity or severalimpurities, in particular the impurities usually present in naturalgraphites.

The present invention also relates to particles based on modifiedgraphite made up of prismatic graphite particles covered with a metallicdeposit and/or a carbonic deposit.

The graphite particles according to the present invention can be used,in particular, as humidity absorbers and/or oxygen absorbers and,especially because of their cycling performance, can be used in themanufacture of negative electrodes, preferably in the manufacture ofnegative electrodes for rechargeable electrochemical generators.

The present invention also concerns methods that make possible thepreparation of these particles and the preparation of electrodescontaining them.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1: According to model 1, transformation of the prismatic particlesinto spherical particles by decreasing the fb and increasing the fe.This transformation can be carried out using several techniques: jetmill, attrition, ball mill, hammer mill, CF mill or atomizer mill,planetary mixer, hybridizer.

According to model 1, transformation of the prismatic particles intospherical particles by keeping fe and fb constant. This transformationis carried out by coating the particles with a metal, polymer or carbon.

FIG. 2: According to model 2, transformation of the cylindricalparticles into spherical particles by decreasing the fb and increasingthe fe. This transformation can be carried out using several techniques:jet mill, attrition, ball mill, hammer mill, CF mill or atomizer mill,planetary mixer, hybridizer.

According to model 2, transformation of the prismatic particles intospherical particles by keeping fe and fb constant. This transformationis carried out by coating the particles with a metal, polymer or carbon.

FIG. 3: According to model 1, the prismatic-shaped artificial or naturalgraphite, in the presence of its impurities and soluble agents of theNaCl and NH₄F type or the like (preferably with spherical shape), istransformed into spherical-shaped graphite by decreasing the basalfraction (fb) and increasing the edge fraction (fe). This transformationcan be carried out using several techniques: jet mill, attrition, ballmill, hammer mill, CF mill or atomizer mill, planetary mixer,hybridizer.

FIG. 4: According to model 2, the carbon fiber, the cylindrical-shapedartificial or natural graphite, in the presence of its impurities andsoluble agents of the NaCl and NH₄F type or the like (preferably withspherical shape), is transformed into spherical-shaped graphite bydecreasing the basal fraction (fb) and increasing the edge fraction(fe). This transformation can be carried out using several techniques:jet mill, attrition, ball mill, hammer mill, CF mill or atomizer mill,planetary mixer, hybridizer.

FIG. 5: According to model 1, the prismatic-shaped artificial or naturalgraphite, in the presence of its impurities and non-soluble agents ofthe SiO₂ and TiO₂ type, ceramic material, hard compound or the like(preferably with spherical shape) is transformed into spherical-shapedgraphite by decreasing the basal fraction (fb) and increasing the edgefraction (fe). This transformation can be carried out using severaltechniques: jet mill, attrition, ball mill, hammer mill, CF mill oratomizer mill, planetary mixer, hybridizer.

FIG. 6: According to model 2, the cylindrical-shaped artificial ornatural graphite, in the presence of its impurities and non-solubleagents of the SiO₂ and TiO₂ type, ceramic material, hard compound or thelike (preferably with spherical shape), is transformed intospherical-shaped graphite by decreasing the basal fraction (fb) andincreasing the edge fraction (fe). This transformation can be carriedout using several techniques: jet mill, attrition, ball mill, hammermill, CF mill or atomizer mill, planetary mixer, hybridizer.

FIG. 7: Comparative modeling between the prismatic and cylindrical modelfor two types of particles of 2 and 40 μm.

FIG. 8: Graphite particles before attrition.

FIG. 9: Graphite particles after attrition.

FIG. 10: Electrochemical results of natural graphite NG20, afterattrition.

FIG. 11: Electrochemical results with commercial spherical graphiteMCMB.

FIG. 12: Scanning electron microscope micrograph showing the potatolikeshape of a 12 μm particle obtained in example 3, according to theinvention.

FIG. 13: Scanning electron microscope (MEB) micrograph showing the trendin basal and edge functions for a graphite particle conforming tomathematical model 1.

FIG. 14: Scanning electron microscope micrograph showing the potatoshape of a 12 μm graphite particle obtained in example 3.

FIG. 15: Micrograph of a particle obtained in example 3, micrographedwith the scanning electron microscope (MEB), showing that the basalfunction (fb) decreases and the function (fe) increases.

FIG. 16: Exemplary particle showing a central layer (0) toward peak (n)or toward base (n) of the particle.

FIG. 17: Exemplary graphite particle is formed by elementary cylindricalparticles with a diameter B and a thickness T.

MODES OF CARRYING OUT THE INVENTION

A first object of the present invention comprises modified graphiteparticles obtained from graphite (preferably from synthetic graphite),the structural parameters of said particles corresponding to at leastone of the equationsfe ₁ =[Y+1]/[(Y+1)+(B/2T)(Y−1)] and fe ₂ =[Y+1]/[(Y+1)+(B/T)(Y−1)],where Y represents a whole number greater than or equal to 1, Brepresents the length of the particle in μm, T represents the thicknessof the particle in μm. These particles also have a potatolike shape andhave at least one of the following two characteristics:

-   -   a tap density, measured according to the method associated with        the instrument sold under the name of Logan Instrument Corp.        Model Tap-2, between 0.3 and 1.5, preferably between 0.5 and        1.4, most preferably between 1 and 1.3 g/cc; and    -   a granulometric dispersion measured according to the method        associated with the particle analyzer sold under the name        Microtac Model X100 Particle Analyzer, such that the D90/D10        distribution ratio varies between 2 and 5 and the particles have        a size between 1 μm and 50 μm, preferably such that the D90/D10        distribution ratio varies between 2.2 and 4.2 and the particles        have a size between 2 μm and 30 μm.

A second object of the present invention comprises modified graphiteparticles obtained from graphite, said particles having a potatolikeshape, comprising impurities in their internal structure and having onthe surface a rate of one or more impurities, measured according to theretrodiffused detector method defined in the publication Kimoto S. andHashimoto H., (1966), in Electron Microphone, John Wiley, New York, page480 and in Gedcke, D. A., Ayers, J. B. and DeNee, P. B. (1978),SEM/1978, SEM Inc, AMF O'Hare, Ill., page 581, that is less than 10%,which preferably varies between 2% and 4%, and said particles alsohaving at least one of the following three characteristics:

-   -   a tap density measured according to the previously identified        method between 0.3 and 1.5, preferably between 0.5 and 1.4, most        preferably between 1 and 1.3 g/cc;    -   a granulometric dispersion measured according to the previously        identified method, such that the D90/D10 ratio varies between        2.2 and 4.2 and the particles have a size between 2 and 30 μm;        and    -   they have, attached to their surface, particles (preferably        potatolike shape, most preferably spherical) of NaCl and/or        NH₄F; preferably the mass of these particles of NaCl and/or of        NH₄F represents 1 to 4% of the total mass of the modified        graphite particles.

The rate of impurities on the surface of graphite may be reduced indifferent ways; one particularly effective method is the one describedin the application PCT/CA100233 held by the Hydro-Québec Company. Thecontents of this document are incorporated in the present application byreference.

A preferred sub-family of the particles according to the second objectof the present application is made up of modified graphite particles forwhich TGA analysis carried out according to the method associated withthe device sold under the name TGA/DTA Model SDT 2960, TA InstrumentsInc., New Castle, Del., gives an initial temperature value between 560and 660 degrees Celsius, associated with the loss of weight, as isillustrated in FIG. 11.

The particle parts of graphite modified according to the invention maycontain impurities, for example, at least one impurity from the groupmade up of the chemical elements Fe, Mo, Sb, As, V, Cr, Cu, Ni, Pb, Co,Ca, Al, Ge, Si, Ba, Be, Cd, Ce, Co, Cu, Dy, Eu, La, Li, Mo, Nd, Ni, Pband Pr.

One preferred sub-family among the graphite particles according to theinvention is made up of particles in which the percentage of impuritiesby weight present in the said particles, expressed with respect to thetotal mass of modified graphite particles and measured according to theash method, is between 1 and 10%, and preferably between 2 and 4%.

More especially interesting are the graphite particles according to theinvention that are substantially lacking in surface impurities andpreferably those devoid of surface impurities.

A third object of the present invention comprises modified graphiteparticles obtained from graphite, said particles having a potatolikeshape and containing from 5 to 20% of at least one of the followingcompounds SiO₂, MgO, ceramic compounds or a mixture of these, saidcompounds preferably being attached to the modified graphite particlesby physical forces and having at least one of the following threecharacteristics:

-   -   a tap density measured according to the previously described        method between 0.3 and 1.5, preferably between 0.4 and 1.4, most        preferably between 1 and 1.3 g/cc;    -   a granulometric dispersion measured according to the previously        defined method, such that the D90/D10 ratio varies between 2 and        5 for particles with a size between 1 and 50 μm, preferably such        that the D90/D10 ratio varies between 2.2 and 4.2 for particles        having a size between 2 and 30 μm; and    -   they have, attached to their surface, particles (preferably        potatolike shaped, most preferably spherical) of NaCl and/or of        NH₄F; preferably the mass of these particles of NaCl and/or of        NH₄F represents 1 to 10% of the total weight of the modified        graphite particles.

One particularly advantageous family of graphite particles according tothe present invention is made up of all the modified graphite particlesin which the interplane distance d₀₀₂ (measured according to the methodassociated with the diffractometer sold under the name XRD AnalysisSiemens Model D500 Diffractometer) varies from 33 to 3.4 angstromsand/or the BET (measured using the method associated with the deviceQuantachrome Autosorb automated gas adsorption system using N₂) variesbetween 0.5 g/m² and 50 g/m².

Among the modified graphite particles of the invention, those having acycling stability greater than 500 cycles are of particular interest inthe scope of electrochemical applications.

A fourth object of the present invention comprises a process forpreparing graphite particles (preferably from natural graphite) that arethe object of the present invention by using at least one physical meansthat makes possible the reduction of at least 50% of the basal function(fb) and the increase of at least 50% of the edge function (fe) of thegraphite particles, such physical means preferably being attrition, ajet mill, ball mill, hammer mill, atomizer mill, in the presence of atleast one chemical compound selected from the group made up of compoundsof the formula MF_(z), in which M represents an alkaline oralkaline-earth metal and z represents 1 or 2 (preferably MFz representsCaF₂, BaF₂, LiF), NaCl and NH₄F or a mixture thereof, said compound orcompounds preferably being added in solid form, preferably at thebeginning of the step using the physical means that makes it possible toreduce the basal function defined by the equation fb=1−fe and toincrease the function fe defined by the ratio (2B/La+T/d₀₀₂):(2B/d₁₀₀+T:d₀₀₂) in which B is the length of the particle, La=d₁₀₀(2n+1)represents d₀₀₂, n represents a number of planes. These functions aredefined and analyzed in detail in the publication Effect of GraphiteParticle Size on Irreversible Capacity Loss, K. Zaghib, G. Nadeau, andK. Kinoshita, Journal of the Electrochemical Society 147 (6) 2110-2115(2000). This document is incorporated into the present application inits entirety by reference.

As described in application PCT/CA0100233 of Hydro-Québec, the use ofNH₄F on the surface the graphite at the time of grinding is veryimportant since NH₄F, at the time of the purification of graphite in thepresence of H₂SO₄ and H₂O, generates HF to dissolve the impurities, inparticular SiO₂.

In the same way, the use of NaCl on the surface of graphite at the timeof grinding is very important, since, at the time of the purification ofgraphite in the presence of H₂SO₄, NaCl generates HCl which alsodissolves the impurities on its surface, in particular the metals Fe, Moand the like.

According to an advantageous mode of using the method, the reduction ofthe basal function and the increase of the edge function, preferably therounding of the particles, is carried out using a means of attrition,said means preferably being made up of balls such as steel balls,ceramic balls or a mixture of steel and ceramic balls.

A fifth object of the present invention comprises a process forpreparing modified graphite particles according to claim 1 or 2,preferably from natural graphite, comprising at least the following twosteps:

-   -   i) modification of the shape of the graphite particles by using        at least one physical means making possible the reduction of at        least 50% of the basal function (fb) and the increase of at        least 50% of the edge function (fe) of synthetic graphite        particles (preferably of natural graphite), such physical means        preferably being attrition (preferably a jet mill, ball mill,        hammer mill, an atomizer mill) in the presence of at least one        chemical compound selected from the group consisting of        compounds of the formula MF_(z), in which M represents an        alkaline or alkaline-earth metal and z represents 1 or 2, MF_(z)        preferably represents CaF₂, BaF₂, LiF or a mixture thereof, the        compound or compounds preferably being added in solid form,        preferably at ambient temperature, preferably at the beginning        of the step using the means that make possible the reduction of        the basal function and the increase of the edge function; and    -   ii) reduction in the amount of surface impurities, preferably by        purification, preferably by chemical purification of the        graphite particles obtained in step i).

According to an advantageous embodiment, the graphite particles used atthe beginning of the process have a size between 1 and 450 μm,preferably between 2 and 350 μm.

According to another advantageous embodiment, the attrition process iscarried out in the presence of an additive, preferably an additive ofthe metallic oxide type such as SiO₂, TiO₂, ZrO₂, and preferably in thepresence of steel balls, ceramic balls or in the presence of a mixtureof steel and ceramic balls.

A preferred variation comprises using the method according to theinvention under conditions such that at least one of the two steps iscarried out in a controlled atmosphere or in air, the controlledatmosphere preferably being based on nitrogen, argon, helium or amixture of these gases.

Step i) can be a hybrid step using both jet milling and attrition, theattrition preferably being carried out after jet milling is used.

According to a particularly advantageous method, step i) of the methodis carried out using jet milling.

A sixth object of the present invention comprises modified graphiteparticles such as defined above and below, as moisture sensors and/oroxygen absorbers.

A seventh object of the present invention comprises negative electrodes,preferably negative electrodes for a rechargeable electrochemicalgenerator prepared with a bonding agent, preferably a bonding agent ofthe PVDF or PTFE type, and with graphite particles according to any oneof the objects of the present invention.

An eighth object of the present invention is made up of a process forpreparing an electrode for a rechargeable generator based on graphiteparticles according to the present invention or based on graphiteparticles such as those obtained by a method according to the invention,comprising at least the following steps:

-   a—solubilization of at least one bonding agent (preferably selected    from the group comprising PVDF, PTFE) in a solvent (preferably in a    strong solvent selected from the group comprising NMP    (N-methylpyrrolidone), cyclopentanone at the highest possible    concentration (preferably greater than 1 g/cc)) to obtain a viscous    solution (A);-   b—coating the viscous solution obtained in the preceding step, which    is a powder-bonding agent compound (B), on a device of the collector    type, preferably on a collector of the metallic type and/or of the    perforated metal collector type, said collector thus treated making    up an electrode; and-   c—drying the electrode prepared in step b); the drying preferably    being carried out using an infrared lamp or using a heating element.

According to a preferred embodiment of this method, in step c) twodistinct means are used in parallel to dry the electrode, these meanspreferably being drying by infrared lamp and drying by heating element.

A ninth object of the present invention is made up of modifiedgraphite-based particles that are made up of prismatic particles ofgraphite covered with a metallic deposit and/or a carbonic deposit, thestructural parameters of said particles corresponding to the equationsfe₁=[Y+1]/[(Y+1)+(B/2T)(Y−1)] and fe_(e)=[Y+1]/[(Y+1)+(B/T)(Y−1)], inwhich: Y represents a whole number greater than or equal to 1, Brepresents the length of the particle in μm, T represents the thicknessof the particle in μm, said particles having a potatolike shape andhaving at least one of the following two characteristics:

-   -   a tap density measured according to the previously defined        method, preferably between 0.3 and 1.5, more preferably between        0.5 and 1.4, and most preferably between 1 and 1.3 g/cc; and    -   a granulometric dispersion measured according to the previously        defined method, such that the D90/D10 ratio varies between 2 and        5 and the particles have a size between 1 and 50 μm, preferably        such that the D90/D10 ratio varies between 2.2 and 4.2 and the        particles have a size between 2 and 30 μm.

Preferably the size of these graphite-based particles is between 1 and50 μm.

According to an advantageous embodiment, these particles have asphericity of 80% or more.

Among all of these graphite-based particles, a preferred sub-family ismade up of particles in which the average thickness of the metallicand/or carbonated coating is between 50 nm and 2 μm.

Another preferred sub-family is made up of graphite-based particles madeup of a coated graphite core, said core making up at least 90% by weightof the total mass of the graphite-based particle, the remaining 10%preferably being made up of at least one metal selected from the groupcomprising Ag, Si, Al and Cu and/or carbon and/or a carbonated polymer,preferably in prismatic or fiber form.

A tenth object of the present invention comprises a process forpreparing the graphite-based particles using prismatic-shaped particles,by coating the particles while keeping the basal function (fb) and theedge function (fe) constant while wrapping the graphite surface with ametallic or carbonic deposit in such a way as to obtain a sphericity of80% or more.

An eleventh object of the present invention comprises a (preferably) insitu process for purifying the surface of graphite particles, by coatingthese particles, in the presence of their impurities, with carbon.

A twelfth object of the present invention comprises the use of modifiedgraphite particles according to the invention in an electrochemicalcell, with a control of the basal function (fb) that permits their usein the presence of an electrolyte based on polyethylene carbonate (PC),the concentration of PC in the electrolyte then being less than 50% byvolume of the electrolytic mixture.

The safety batteries resulting from this utilization also make up anobject of the present invention.

A thirteenth object of the present invention is made up of the use ofthe graphite-based particles according to the invention with a constantbasal function (fb) which makes possible their use in the presence of anelectrolyte based on polyethylene carbonate (PC), up to a PCconcentration in the electrolyte that is then less than or equal to 100%by volume of the electrolytic mixture. The batteries resulting from thisutilization are safe and make up an object of the present invention.

Thus within the scope of the present invention, in particular, a newmethod for transforming the natural graphite particles into sphericalparticles is described. The coarse graphite powder, in the presence ofthese impurities that play a microabrasive role (HQ patent), havinginitial particles of 375 μm, is subject to grinding by attrition inorder to reduce its size to a d₅₀ of 10 μm. Steel balls are added to thegraphite powder in a weight ratio of 1:10 graphite: balls. Grinding(ATTRITOR, type B, size S Union Process Inc, AKRON) is accelerated to aspeed of 400 rpm for 60 minutes. After 60 minutes, the grinding isstopped and an evaluation of the granulometry and of the specificsurface area is carried out on the sample. If the desired granulometricdistribution is not achieved, the grinding is repeated for a period of10 minutes. These steps will be continued until a d₅₀ of 10 μm isobtained. A second 500 g sample is ground in an Alpine jet air grinderto obtain a d₅₀ of 10 μm. A comparative study using scanning microscopyis carried out on the two samples after grinding. This allows us toidentify whether the shape of the flakes obtained by attrition comescloser to that of a sphere. We have also used hybrid grinding; the sizeof the particle is reduced first to 20 μm by jet milling, then theparticle is sized to 10 μm using attrition.

The use of spherical graphite as anode in a rechargeable batteryconfiguration has many advantages in comparison to graphite in flakes,in particular:

-   -   the density of the consistency is increased;    -   the coating is more uniform;    -   the porosity is reduced;    -   the fraction of basal planes is reduced;    -   better inter-particle electrical contact;    -   decomposition of the electrolyte is reduced;    -   a rapid charge-discharge rate;    -   the intercalation kinetics are better; and    -   the safety of the battery is improved.

Application of this process to natural graphite improves itselectrochemical performance and its use for coating the electrode.Natural graphite made into spherical form combines the advantages of thetwo carbons: those of natural graphite and those of spherical artificialgraphite. The energy is maintained at its maximum with natural graphite(average capacity and voltage). The contribution of the basal planes isreduced, which promotes on the one hand the reduction in irreversiblecapacity due to passivation and the increase of diffusional parts(edges) along the crystallographic axis C (perpendicular to the planesformed by the carbon atoms). In addition, the problem of anisotropy isreduced and the intercalation kinetic is improved. Spherical particlesmake coating of the electrodes more homogeneous and make the electrodesobtained less porous. The thickness of the electrodes with sphericalparticles is better controlled and can achieve smaller thicknesses forpower applications, such as pulses for telecommunication and powertake-offs for hybrid vehicles. These characteristics facilitate thedesign of super-thin Li-ion batteries up to the level of polymerbatteries.

Calculation models of the relationship between the graphite particlesand their surfaces (basal and edge): comparison between the prismaticand spherical structure.

Model I

A mathematical model for the spherical particle has been developed inorder to express the relationship between the size of the graphitecrystallite and the sites on the surface by using crystallographicparameters a, b and c. In this model, it is assumed that the sphere isformed of prismatic layers with dimension Ai.Bi (basal plane) andthickness T (edge) stacked on each other.

It is also assumed that parameters A and B are smaller by the samefactor (Y) as they move away from the central layer (0) toward peak (n)or toward base (n) of the particle (FIG. 16).

Central layer (0): A, B, T

Layer (1):

A₁, B₁, T where A=Y A₁, B=Y B₁ and Y 1A ₁ =A/Y, B ₁ =B/Y  (1)Layer (2):A₂, B₂, T where A₁=Y A₂, B₁=Y B₂A ₂ =A ₁ /Y=A/Y ² , B ₂ =B ₁ /Y=B/Y ²  (2)Layer (3):A₃, B₃, T where A₂=Y A₃, B₂=Y B₃A ₃ =A ₂ /Y=A/Y ³ , B ₃ =B ₂ /Y=B/Y ³  (3)

-   -   .    -   .    -   .        Layer (n):        A_(n), B_(n), T where A_(n-1)=Y A_(n), B_(n-1)=Y B_(n)        A _(n) =A _(n-1) /Y=A/Y ^(n) , B _(n) =B _(n-1) /Y=B/Y ^(n)  (4)

Thus, for the surface of the edge:EA=2T[(A+B)+(A ₁ +B ₁)+(A ₂ +B ₂)+(A ₃ +B ₃)+ . . . +(A _(n) +B_(n))]  (5)By substitution of the values of A₁, B₁, A₂, B₂, . . . , A_(n), B_(n) ofequations 1, 2, 3 and 4 in 5;EA=2T(A+B)[1+1/Y+1/Y ²+1/Y ³+ . . . +1/Y ^(n)]=2 T(A+B)Σ1/Y ^(i) (i=0 to n)  (6)

Considering the symmetry of the sphere, equation (6) will take the form:EA=4T(A+B)[Σ1/Y ^(i)]−2T(A+B), (i=0 to n)  (7)

The series [Σ1/Y^(i)] (i=0 to n) converges toward the term Y/(Y−1)

(mathematic table 1+x+x²+ . . . +x^(n)=1/(1−x), where 1<x<1)EA=4T(A+B)Y(Y−1)⁻¹−2T(A+B)EA=2T(A+B)[2Y(Y−1)⁻¹−1]  (8)

For the surface of the basal planes:BA=2[(AB−A ₁ B ₁)+(A ₁ B ₁ −A ₂ B ₂)+(A ₂ B ₂ −A ₃ B ₃)+ . . . (A _(n-1)B _(n-1) −A _(n) B _(n))]=2[AB−A _(n) B _(n)]=2 AB[1−1/Y ^(2n)]BA=2AB[1−1/Y ^(2n)]  (9)The total surface area will be:S _(t) =EA+BA=2T(A+B)[2Y(Y−1)⁻¹−1]+2AB[1−(1/Y ^(2n))]The fraction of the edge sites (f_(e));f _(e) =EA/S _(t)=2T(A+B)[2Y(Y−1)⁻¹−1]/[2T(A+B)[2Y(Y−1)⁻¹−1]+2AB[1−(1/Y ^(2n))]]f _(e) =[Y+1]/[(Y+1)+[AB/T(A+B)][1−(1/Y ^(2n))](Y−1)]  (10)

When A=B;f _(e) =[Y+1]/[(Y+1)+(B/2T)(1−Y ^(−2n))(Y−1)]  (11)

1) in an ideal case, when n→∞, Y^(−2n)→0;

(when an infinite number of prismatic layers are considered)f _(e) =[Y+1]/[(Y+1)+(B/2T)(Y−1)]  (12)

equation (12) expresses the relationship between the surface of theedges as a function of the dimensions of the particle and of parameterY.

Model 2

If it is considered that the approximation of the graphite particle isformed by elementary cylindrical particles with a diameter B and athickness T (FIG. 17).

The edge and basal surfaces are defined by:EA=πBTBA=πB ²/2hence fraction f_(e)f _(e) =[Y+1]/[(Y+1)+(B/T)(1−Y ^(−2n))(Y-1)]  (13)

1) When n→∞, Y^(−2n)→0f _(e) =[Y+1]/[(Y+1)+(B/T)(Y−1)]  (14)

In these two models, it is possible to see that the trend in the basaland edge planes of the prismatic shape toward the spherical shape causesa noticeable decrease of the surface area of the basal planes. Thisincreases the edge surface fraction in comparison to the total surfacearea of the particle. From the electrochemical point of view,passivation will be reduced with the reduction of the basal surface andon the other hand, intercalation will be more accessible over a largeedge surface. For a given value of parameter (Y), where Y≧1, it ispossible to compare the fraction of the edge surface f_(e) obtained inequations 12 and 14. Tables 1.a-d present the results obtained for theedge fraction (f_(e)) calculated for the two approximations.

TABLE 1.A Y = 1.001 Particle B(μ) size basal T(μ) Gap (%) (μ) lengthedge length f_(e) prismatic f_(e) cylindrical (f_(e)p-f_(e)c) 2 2 0.210.99763 0.99526 0.2363 12 12 0.49 0.99392 0.98791 0.60089 20 20 1.540.99677 0.99355 0.32139 30 30 2.03 0.99632 0.99267 0.36523 40 40 2.850.99651 0.99303 0.34706

TABLE 2.B Y = 1.01 B(μ) Particle basal T(μ) Gap (%) size (μ) length edgelength f_(e) prismatic f_(e) cylindrical (f_(e)p-f_(e)c) 2 2 0.210.97686 0.95476 2.2096 12 12 0.49 0.94258 0.89139 5.1185 20 20 1.540.96871 0.93931 2.9396 30 30 2.03 0.96454 0.93151 3.303 40 40 2.850.96626 0.93473 3.1533

TABLE 3.C Y = 1.1 B(μ) Particle basal T(μ) Gap (%) size (μ) length edgelength f_(e) prismatic f_(e) cylindrical (f_(e)p-f_(e)c) 2 2 0.210.81516 0.68799 12.717 12 12 0.49 0.63168 0.46164 17.003 20 20 1.540.76382 0.61788 14.593 30 30 2.03 0.73972 0.58695 15.277 40 40 2.850.74953 0.5994 15.013

TABLE 4.D Y = 1.5 B(μ) Particle basal T(μ) Gap (%) size (μ) length edgelength f_(e) prismatic f_(e) cylindrical (f_(e)p-f_(e)c) 2 2 0.21 0.51220.34426 16.793 12 12 0.49 0.28994 0.16955 12.039 20 20 1.54 0.435030.27798 15.705 30 30 2.03 0.40358 0.2528 15.078 40 40 2.85 0.416060.26267 15.339

The best approximation is normally obtained with a value of parameter Yclosest to the unit. For Y=1.001, the value of f_(e) in theapproximations converges toward one, independently of the size of theparticles. This means that the surface of basal planes I tends towardzero, which is the ideal case (Table 1a).

The gap between the two approximations increases with Y, as well as theeffect of the particle size. When Y=1.01, there is a 3% gap between thetwo approximations and less than 2% between the size of the particles.While at Y=1.5, the gap is around 16%, while that between the particlesremains between 2-3%.

This variation in factor f_(e) depends on the way the shape of theelementary particles is considered, as well as the step between them(Y), which will form the final spherical particle. The divergence off_(e) from the unit gives the fraction of basal planes. In fact, thespherical form of the natural graphite particles is more advantageousand makes it possible to have a more rapid intercalation rate and lessirreversible capacity (less basal surface).

2) When the term (1−Y^(−2n)) is considered in equations (11) and (13);if Y=1.1, the two equations rapidly converge (FIG. 3) after n iterations(n<50). As shown in FIG. 7, the results obtained show that:

-   -   the particles with small size are the easiest to make spherical        (fe(2 ìm)>fe(40 ìm))    -   a prismatic approximation gives higher values for f_(e).

The following examples, which are given purely by way of illustration,should not be interpreted as constituting any limitation of the presentinvention.

Example 1 Preparation of Potatolike Shaped Modified Graphite Particlesfrom Natural Graphite Using Attrition

Natural graphite is used that has an initial particle size of 375 μm, apurity rate 98% and in the shape of flakes. The specific surface area ofthis graphite is about 1 m²/g. The natural graphite powder is groundusing an “attrition” process in order to transform these particles intospherical particles.

The d₀₀₂ has not changed after a change to spherical shape and has avalue of 3.36 angstroms. Analysis using scanning electron microscopy(SEM) has shown, in micrograph 1 a (FIG. 8) compared to micrograph 1 b(FIG. 9) before attrition, the change in the shape of the particleswhile the size is essentially maintained at the same scale.

The fluorinated polyvinylidene PVDF bonding agent is solubilized in NMPN-methylpyrrolidone. An 80:20 mixture of the solvents acetone/toluene isadded to the PVDF-NMP paste to form the coating composition. The naturalgraphite powder transformed into spheres is dispersed in the coatingcomposition in a weight ratio of 90:10. This mixture is applied on acopper collector using the doctor blade method. The electrode is driedusing an infrared lamp. The electrode is mounted in a 2035 button-typebattery. A Celgard™ 2300 separator soaked with electrolyte 1MLiPF₆+EC/DMC: 50:50 (ethylene carbonate+dimethyl carbonate) is used.

Electrochemical tests were carried out at ambient temperature. Thecharge curves were obtained between O and 2.5 volts in C/24 for twobutton cells, P1 and P2 (FIG. 4), FIG. 10. The reversible capacity is370 mAh/g. This result is comparable to that obtained with electrodesprepared using standard natural graphite in flake form, as well asartificial graphite in spherical form (MCMB28-25).

Example 2 Preparation of Potatolike Shaped Modified Graphite Particlesfrom Natural Graphite Using Jet Milling

Natural graphite is used comprising particles with initial size of 375μm, purity rate 98% and in the shape of flakes. The specific surfacearea of this graphite is around 1 m²/g. In a first step, the particlesare reduced to 20 μm by jet milling. In a second step, the particles arecut to 10 μm in spherical shape by attrition.

Example 3 Preparation of Potatolike Shaped Modified Graphite ParticlesUsing Jet Milling in the Presence of NH₄F

20 kg of Brazilian graphite with an average particle size (d₅₀) of 350μm and a purity of 98.5% are mixed in a reactor with 10% by weight ofNH₄F.

To homogenize the mixture, the jar mill method was used with ceramicballs having a diameter of 50 mm, for 24 hours. This mixture is groundby jet milling, the air pressure in the jet mill fluctuating between 100and 125 psi during processing.

At the end of the processing, the average size of the particles isreduced to between 10 and 20 μm and the particles obtained have theshape of a potato.

FIG. 12, which is a scanning electron microscope micrograph, clearlyshows the potatolike shape with a 12 μm particle.

FIG. 13, which is a scanning electron microscope micrograph, clearlyshows that the basal function (fb) decreases and the edge function (fe)increases, thus the graphite planes at the basal level join the graphiteplanes at the level of the edge in the form of a saw tooth (verificationof mathematical model 1).

Example 4 Preparation of Potatolike Shaped Modified Graphite Particlesby Attrition and in the Presence of NaCl

20 kg of Brazilian graphite with an average particle size (d₅₀) of 350μm and a purity of 98.5% is mixed in a reactor with 10% by weight ofNaCl.

Homogenization of the mixture is carried out using the jar mill methodwith ceramic balls having a diameter of 50 mm, for 24 hours.

This mixture is ground by jet milling. The dwell time of the mixture inthe chamber is 45 minutes.

During processing, air pressure in the jet mill fluctuates between 100and 125 psi. In the course of processing, the size of the particles isreduced to between 10 and 20 μm and the shape of the particles obtainedis potatolike.

The scanning electron microscope micrograph (MEB), FIG. 14, clearlyshows the potatolike shape obtained with a 12 μm particle.

The scanning electron microscope micrograph (FIG. 15) clearly shows thatthe basal function (fb) decreases and that the edge function (fe)increases, a result of the rounding of the particle (verification ofmathematical model 1).

Example 5 Preparation of Particles Based on Graphite Having a CoreCoated with a Layer of Carbonated Cellulose Carbonate

In a 200 ml container, a mixture is prepared of 2 g of Braziliangraphite with an average particle size (d₅₀) of 20 μm and prismaticshape, and 10% cellulose acetate.

The mixture is dissolved in acetone and homogenized using the ball millmethod. The mixture is processed at 400° C. for 3 hours in a nitrogenatmosphere.

The particles obtained are potatolike shaped.

One of the advantages of this treatment is that the carbonated layerobtained on the surface plays the role of purifier since it covers allthe impurities existing at the surface.

Example 6 Preparation of Graphite-Based Particles Having a Core Coatedwith a Layer of Carbonated PE-PEO-Glycol

In a 200 ml reactor, a mixture of 2 g of Brazilian graphite with anaverage particle size (d₅₀) of 20 μm and prismatic shape, and 10% of thePE-PEO-glycol compound is prepared.

The mixture is dissolved in acetone, then it is homogenized by ballmilling. The mixture is processed at 400° C. for 3 hours in a nitrogenatmosphere.

The particles obtained are potatolike shaped.

One of the advantages of this treatment is that the carbonated layerobtained on the surface plays the role of a purifier since it covers allthe impurities present at the surface.

Example 7 Preparation of Graphite-Based Particles Having a Core Coatedwith a Layer of Silver

A Brazilian graphite with an average particle size (d₅₀) of 20 μm andprismatic shape, with a purity of 98.5%, is covered on its surface witha 10% by weight silver deposit.

The deposit is obtained by evaporation, using an Edwards Coating SystemModel E306A evaporator.

The reversible capacity is 387 mAh/g, 15 mA/g more than the theoreticalcapacity of natural graphite.

A low specific surface area is associated with a lower passivation filmcontribution. Within the scope of the present invention, it has thusbeen established that this passivation layer forms on the basal part(organic species): ICL_(basal) and on the edge part (inorganic species):ICL_(edge). In summary, ICL_(basal) is 40 times higher than ICL_(edge).This shows that the decrease in the basal function is very important inorder to reduce the irreversible capacity and the exhaust of gases. Thisis associated with the safety of the battery.

The invention claimed is:
 1. A process for preparing modified graphite-based particles comprising: coating a surface of prismatic-shaped graphite-based particles with a metallic deposit or with a metallic and carbonic deposit while keeping a basal function (fb) and an edge function (fe) of the prismatic-shaped graphite-based particles constant, wherein the coating of the surface of the prismatic-shaped graphite-based particles results in a sphericity of 80% or more.
 2. The method of claim 1, wherein the modified graphite-based particles comprise at least one characteristic selected from the group consisting of a tap density between 0.3 and 1.5 g/cc; a granulometric dispersion such that a D90/D10 ratio varies between 2 and 5; and a size between 1 and 50 μm.
 3. The method of claim 1, wherein the coating of the surface of the prismatic-shaped graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises disposing the graphite-based particles in a solvent with a metallic and/or carbonic source followed by homogenizing by milling.
 4. The method of claim 3, wherein the milling comprises ball milling.
 5. The method of claim 1, wherein the coating of the surface of the prismatic-shaped graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises evaporating a metallic and/or carbonic source onto the surface of the prismatic-shaped graphite-based particles.
 6. A process for preparing modified graphite-based particles comprising: coating a surface of natural graphite-based particles with a metallic deposit or with a metallic and carbonic deposit while keeping a basal function (fb) and an edge function (fe) of the graphite-based particles constant, wherein the coating of the surface of the natural graphite-based particles results in a sphericity of 80% or more.
 7. The method of claim 6, wherein the modified graphite-based particles comprise at least one characteristic selected from the group consisting of a tap density between 0.3 and 1.5 g/cc; a granulometric dispersion such that a D90/D10 ratio varies between 2 and 5; and a size between 1 and 50 μm.
 8. The method of claim 6, wherein the coating of the surface of the natural graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises disposing the graphite-based particles in a solvent with a metallic and/or carbonic source followed by homogenizing by milling.
 9. The method of claim 8, wherein the milling comprises ball milling.
 10. The method of claim 6, wherein the coating of the surface of the natural graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises evaporating a metallic and/or carbonic source onto the surface of the natural graphite-based particles.
 11. A process for preparing modified graphite-based particles comprising: coating natural graphite-based particles with a metallic deposit or with a metallic and carbonic deposit, wherein the modified graphite-based particles, prior to coating, have a first basal function (fb) and a first edge function (fe), wherein the modified graphite-based particles have a second basal function (fb) and a second edge function (fe), wherein the second edge function is greater than the first edge function and the second basal function is smaller than the first basal function.
 12. The method of claim 11, wherein the modified graphite-based particles comprise at least one characteristic selected from the group consisting of a tap density between 0.3 and 1.5 g/cc; a granulometric dispersion such that a D90/D10 ratio varies between 2 and 5; and a size between 1 and 50 μm.
 13. The method of claim 11, wherein the coating of the surface of the natural graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises disposing the graphite-based particles in a solvent with a metallic and/or carbonic source followed by homogenizing by milling.
 14. The method of claim 13, wherein the milling comprises ball milling.
 15. The method of claim 11, wherein the coating of the surface of the natural graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises evaporating a metallic and/or carbonic source onto the surface of the natural graphite-based particles.
 16. A process for preparing modified graphite-based particles comprising: coating prismatic-shaped graphite-based particles with a metallic deposit or with a metallic and carbonic deposit, wherein the prismatic-shaped graphite-based particles, prior to coating, have a first basal function (fb) and a first edge function (fe), wherein the modified graphite-based particles have a second basal function (fb) and a second edge function (fe), wherein the second edge function is greater than the first edge function and the second basal function is smaller than the first basal function.
 17. The method of claim 16, wherein the modified graphite-based particles comprise at least one characteristic selected from the group consisting of a tap density between 0.3 and 1.5 g/cc; a granulometric dispersion such that a D90/D10 ratio varies between 2 and 5; and a size between 1 and 50 μm.
 18. The method of claim 16, wherein the coating of the surface of the prismatic-shaped graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises disposing the graphite-based particles in a solvent with a metallic and/or carbonic source followed by homogenizing by milling.
 19. The method of claim 18, wherein the milling comprises ball milling.
 20. The method of claim 16, wherein the coating of the surface of the prismatic-shaped graphite-based particles with the metallic deposit or with the metallic and carbonic deposit comprises evaporating a metallic and/or carbonic source onto the surface of the prismatic-shaped graphite-based particles. 